Propeller type marine propulsion systems have been in use for many years, dating back to the earliest steam driven ships. Throughout its long history, the propeller or screw configuration has undergone successive improvements in the interest of better performance and higher operating efficiencies. After all such improvements, however, the direct drive propeller remains a less than optimum propulsion means.
The major problem with the propeller drive is that high operating efficiency and optimum performance can only be achieved over a narrow range of operating conditions. For example, a propeller that is designed with a low pitch and large blades will serve well to propel a tug boat with its heavy loads at low speeds. But for a pleasure boat or a speed boat a smaller propeller with a higher pitch is needed. In nearly every case, the propeller design is a compromise, with trade-offs and sacrifices in performance or efficiency at both ends of the operating range. At startup from a dead-in-the-water condition, a large short-pitch propeller is desired, while for cruising at high speed a longer pitch and a smaller propeller length is indicated. The solution is typically somewhere in between too short a pitch and too small a propeller at start-up and too long a pitch and too large a propeller for cruising. Racking the engine at start-up can readily produce cavitation which significantly reduces thrust and results in excessive fuel use. The slow forward motion of the craft at start-up with the attendant low head pressure accounts for such cavitation. At cruising speeds, on the other hand, the increased availability of water to the propeller and the resulting increased pressure under this condition could readily supply a propeller with a longer pitch and a higher drive capability.
The jet or turbine drive has been applied to speed boats with remarkable success in terms of performance. High acceleration at low speeds as well as high power at high speeds are readily achieved in such drives, but at the expense of low operating efficiencies.
Similar problems have been encountered in connection with aircraft propulsion systems. In the conversion from propeller drives to jet engines it soon became apparent that the jet engine in its simplest form was not as fuel efficient as it needed to be, especially when fuel costs began to rise during the early stages of the energy crises.
The high exhaust velocity of the jet engine accounts for its inefficiency. More thrust is obtained from a large mass of relatively slow moving air than from a smaller stream of fast-moving air.
Engine designers have found various answers to these problems in the form of turbofan or fanjet engines in which the fans normally used for compression are enlarged to supply additional air that bypasses the combustion engine and then mixes with the exhaust gases to form an enlarged exhaust stream. Considerably higher operating efficiencies have been obtained in this way. More recently, further improvements have been realized using combinations of turbines and turbine-driven propellers. Examples are the propfans and, in particular, the unducted fan produced by the General Electric Company.
Similar innovations are needed in the art of marine propulsion systems if a truly universal drive is to be achieved with both high efficiency and high performance over a wide range of operating conditions.